1. Field of the Invention
The present invention relates to an electrophotography image forming apparatus, such as a copying machine, a facsimile, and a printer; an image forming method used therein; and a computer program product for causing a computer to execute the method.
2. Description of the Related Art
Electrophotography image forming apparatuses have been widely used in high speed mass printing field typified by newspapers, posters, books, and direct mail with recent progress in high speed operation and colorization technology for the apparatuses. In such field, various kinds of printing media are used. There are increasing demands for printing on a transfer target having low surface smoothness such as embossed paper. However, when the electrophotography image forming apparatuses are used for such medium, a problem arises in that toner is not sufficiently transferred to a valley portion compared with a ridge portion of a transfer target; because a transfer electric field at the valley portion is smaller than that at the ridge portion. As a result, white spots occur in an image formed on a transfer target having low surface smoothness. In order to address this problem, image forming apparatuses employing transfer methods for solving the problem have been proposed. Refer to Japanese Patent Application Laid-open No. 2002-156839, Japanese Patent Application Laid-open No. 2002-202638, and Japanese Patent Application Laid-open No. 2006-267486. For example, Japanese Patent Application Laid-open No. 2006-267486 discloses an image forming apparatus employing a transfer method in which an alternating voltage is superimposed on transfer bias. In the image forming apparatus, the superimposing of the alternating voltage on the transfer bias causes toner to be reciprocated between an image carrier and a transfer target so as to increase frequency of toner contacting with a valley portion of a recording medium, thereby reducing failures of toner transfer to a valley portion on a surface of the recording medium. This image forming apparatus is suitable for printing characters on rough surface paper, such as in a case of newspaper printing, because the reduction of failures of toner transfer to a valley portion enhances character visibility.
In most cases where embossed paper having ridges and valleys that are intentionally provided to give premium accents is used as a transfer target, images having higher design property rather than characters are printed. In such cases, printing that utilizes texture of the transfer target is desired. For example, when texture of an image drawn with colored pencils or printed by block print is desired, a toner adhesion amount on a valley portion is reduced; while when texture of an image drawn with paints is desired, a toner adhesion amount on a valley portion is set to be larger than that on a ridge portion. In this way, various kinds of texture can be made by intentionally differentiating the toner adhesion amounts between ridge and valley portions. The prior art image forming apparatuses, however, cannot adjust the toner adhesion amount between ridge and valley portions. As a result, a problem arises in that texture of a transfer target is lost and not utilized.